Method of preparing a metal powder consisting at least substantially of iron for magnetic recording

ABSTRACT

Iron powders (which may contain, in addition, nickel and/or cobalt) to be used for magnetic recording are prepared according to the invention by reducing the metal oxidic or metal hydroxidic starting material in a finely divided state with a gaseous reduction agent (preferably hydrogen) at a pressure of at least 10 atm. The advantage hereof is that the reduction is effected at an acceptable rate at comparatively low temperatures at which sintering of the formed metal particles does not yet occur. Actually, such a sintering unfavorably affects the properties of the powders in question with a view to their usability for magnetic recording.

United States Patent 1 Inventors Al Au omat n Gi ti Freerk IIuizinga, both of Emmasingel, Eindhoven, Netherlands {21 Appl. No. 802,679 [22] Filed Feb. 26, 1969 [45] Patented Sept. 21, 1971 [73] Assignee U.S. Philips Corporation New York, N.Y. [32] Priority Mar. 5, I968 [33] Netherlands [3 1] 6803123 [54] METHOD OF PREPARING A METAL POWDER CONSIS'I'ING AT LEAST SUBSTANIIALLY OF IRON FOR MAGNETIC RECORDING 2 Claims, No Drawings [52] U.S. Cl 75/0.5 BA, 148/105 [51] Int. Cl IIOII 1/06, B22f 9/00 [50] Field oI Search 75/0.5 AA,

[56] References Cited UNITED STATES PATENTS 3,005,701 10/1961 Eberhardt 75/0.5 BA 3,347,659 l0/l967 Volk et al.

Primary E.raminerl.,. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Frank R. Trifari ABSTRACT: Iron powders (which may contain, in addition, nickel and/or cobalt) to be used for magnetic recording are prepared according to the invention by reducing the metal oxidic or metal hydroxidic starting material in a finely divided state with a gaseous reduction agent (preferably hydrogen) at a pressure of at least 10 atm. The advantage hereof is that the reduction is effected at an acceptable rate at comparatively low temperatures at which sintering of the formed metal particles does not yet occur. Actually, such a sintering unfavorably affects the properties of the powders in question with a view to their usability for magnetic recording.

METHOD OF PREPARING A METAL POWDER CONSISTING AT LEAST SUBSTANTIALLY OF IRON FOR MAGNETIC RECORDING The invention relates to a method of preparing a metal powder consisting at least substantially of iron, for magnetic recording.

It is known to prepare metal powders of the above mentioned type which may consist of an alloy of iron with nickel and/or cobalt by reduction of the metal oxides and/or metal hydroxides by means of hydrogen or another gaseous reduction agent at a pressure of 1 atm. However, the reduction then proceeds very slowly. in order to perform the reduction at a rate which is acceptable for practical purposes, it must be carried out at temperatures above 300 C. However, this exhibits the drawback that the formed metal particles sinter so that a product is formed which is not or at least less readily suitable as a material for magnetic recording.

The invention removes the above-mentioned drawback in that the reduction is carried out at a temperature not exceeding 300 C. and at a pressure of the gaseous reduction agent of at least atm. As a result of this a reduction speed is obtained which is acceptable for practical purposes in every respect, while nevertheless the temperature at which the reduction is effected, remains below the level at which the above-mentioned sintering of the formed metal particles begins to occur.

The reduction may be promoted by withdrawing water which is formed during the reaction from the gas atmosphere and the reaction space. Therefore the reduction is preferably carried out in the presence of a water-binding agent in the reaction space. A suitable water-binding agent is, for example,

calcium oxide, CaO. Other water-binding substances, for example, phosphorus pentoxide, P,O may also be used.

it was already known to reduce metal oxides to metals by means of gaseous reduction agents, for example, hydrogen and carbon monoxide, at pressures exceeding 1 atm. (see .I. Tumarev and L. A. Panyushin, Trudy Soveshchaniya Sozvannogo lnstitutom Metallurgii Uralskogo Filiala Akademii Nauk SSSR i Magintorgorskim Metallurgicheskim Kombination, 1955, pages ill-127, the same authors, Stall8, pages 769-776 (1958) and Yu.P. Svintsov, Trudy Leningradskogo Politekhnicheskogo instituta imeni H. l. Kalinina, i960, No. 12, PP 81-91).

However, these authors do not describe pressures exceeding 7 atm., while any usefulness of the resulting reduction products as a material for magnetic recording is not described either.

in order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the ensuing specific examples.

EXAMPLE I Two porcelain crucibles were placed in an autoclave, capacity 500 ml. in the lowermost crucible 30 g. of CaO were provided while the uppermost crucible contains 'y-Fe O in the form of accicular particles the largest dimension of which was smaller than 1 micron and a ration (length/thickness) of approximately 5. The autoclave was rinsed three times with hydrogen, then filled with the desired quantity of hydrogen, followed by heating at a temperature of maximally 300 C. which temperature was maintained for 4 hours. After cooling, the autoclave was evacuated after which so much ethanol was provided in the crucible containing the formed iron that this iron was entirely immersed. Said crucible was then taken out of the autoclave. The ethanol was decanted from the crucible after which the iron was dried in air.

The following table A provides a survey of the values measured in the resulting iron powder compositions of the saturation magnetization (o' s), which is to be understood to mean the magnetic moment per kilogram in a field of i0 amp./m. expressed in volt-sec. m./kg. and of the product (p xfli of thema etic permeabil ity, p.,, (i.e. the ratio B/H of the magnettc ux density in volt-seconds per square meter and the magnetic field strength in amp./m, everything in vacuo), and the magnetization coercive force, ,l-h.

It is to be noted that the quantities of iron oxide to be reduced are slightly adapted to the'hydrogen pressure used in that sense that said quantities are larger as the pressure of hydrogen is higher, this with the object of varying the ratio of the quantities of iron oxide and hydrogen not too much.

For comparison may serve that the theoretical value of (a, X10) for pure iron is 2.75 volt-sec. m./kg.

EXAMPLE II The performance of the experiments differs from those described in example I only in that in these experiments an iron oxide composition consisting of a--Fe,0;,particles in the form of hexagonal dipyramids having a longitudinal axis of approximately 800 A were used. Table B below gives a survey of the values measured in the resulting iron powder compositions of the saturation magnetization, 0,, and of the product ,H,.) of the magnetic permeability, n and the magnetiza tion coercive force ,l-l

TABLE 13 Quantity 0! iron Reaction Initial temperature pressure in, nXlO uQXrHEXIO in, C. atmospheres (Vsec.m./kg.) (Vsee/mfl) in the same manner nickel and/or cobalt-containing iron powders with analogous properties can be obtained.

What is claimed is:

l. A method of preparing iron powder suitable for magnetic recording, comprising the step of reducing powdered iron oxide the particles of which have a largest dimension not exceeding in at a temperature not exceeding at 300 C. in hydrogen at a pressure of at least 10 atm.

2. A method as claimed in claim 1, wherein the iron oxide is reduced in the presence of a water-binding agent. 

2. A method as claimed in claim 1, wherein the iron oxide is reduced in the presence of a water-binding agent. 